1. Field of the Invention
The invention relates to the use of vinylaromatic/1,3-diene copolymers stabilized with protective colloids and in the form of their aqueous polymer dispersions or water-redispersible polymer powders as adhesive for porous substrates.
2. Background Art
WO-A 96/37551 discloses wood adhesives which comprise a mixture of a polymer containing hydroxyl groups, in particular polyvinyl alcohol, and a carboxyl-substituted styrene polymer, in particular styrene/butadiene latex, and a polyfunctional aziridine as the crosslinking agent for hydroxyl and carboxyl groups. When these polymers are used as wood adhesives, however, there is the disadvantage that, because of their high reactivity in the ready-to-use state, these cannot be used as storage-stable one-component systems.
JP-A 1/229085 (Derwent Abstract AN 89-312288) discloses wood adhesives which are composed of a chelating agent and a dissolved or dispersed adhesive. Polymer dispersions based on vinyl acetate, acrylate or styrene/butadiene, in addition to condensation resins, are mentioned here in particular as adhesives. These can optionally contain polyvinyl alcohol as a dispersing agent. Dibasic acids, such as oxalic or malonic acid, in addition to oxocarboxylic acids, and salts thereof, are mentioned as chelating agents.
JP-A 58/162645 (Derwent Abstract AN 83-805435) discloses adhesives for gluing porous materials, for example wood. The adhesives consist of a mixture of a vinyl acetate/ethylene copolymer dispersion and a styrene/butadiene latex modified with carboxyl groups, the dispersions mentioned being stabilized by emulsifiers. A problem with emulsifier-stabilized dispersions is their unsatisfactory resistance to water.
JP-A 60009970 discloses an adhesive composition for carpet bonding, containing glycine in order to prevent bubbling. The binder is again an emulsifier-stabilized latex with the drawback of unsatisfactory water resistance.
The invention was thus based on the object of providing adhesives which are distinguished by improved hydrophobicity and resistance to water compared with the known adhesives for porous materials, with a simultaneously good storage stability.
The invention relates to the use of vinylaromatic/1,3-diene copolymers, having a glass transition temperature Tg of xe2x88x9270xc2x0 C. to +100xc2x0 C., stabilized with protective colloids and in the form of their aqueous polymer dispersions or water-redispersible polymer powders as adhesives for porous substrates, the polymer dispersions and the polymer powders being obtained by emulsion polymerization of a mixture comprising
a) 20 to 80% by weight of one or more vinylaromatics,
b) 19.9 to 79.9% by weight of one or more 1,3-dienes,
c) 0.1 to 15% by weight of one or more comonomers from the group consisting of N-methylolacrylamide, N-methylolmethacrylamide and the N-(alkoxymethyl)-acrylamides and N-(alkoxymethyl)methacrylamides with a C1- to C6-alkyl radical, the data in % by weight adding up to 100% by weight,
in the presence of one or more protective colloids and with the exclusion of emulsifier, and optionally drying the aqueous polymer dispersion obtained by this process.
Suitable vinylaromatics are styrene and methylstyrene, and styrene is preferably copolymerized. Examples of 1,3-dienes are 1,3-butadiene and isoprene, and 1,3-butadiene is preferred. The content of comonomers a) and b) is preferably in each case 30 to 70% by weight, based on the total weight of the comonomers.
Up to 30% by weight, based on the total weight of the monomer phase, of further monomers which can be copolymerized with vinylaromatics and 1,3-dienes, such as vinyl chloride, (meth)acrylic acid esters of alcohols having 1 to 15 C atoms or vinyl esters of unbranched or -branched carboxylic acids having 1 to 15 C atoms, can optionally also be copolymerized.
0.05 to 10% by weight, based on the total weight of the monomer mixture, of auxiliary monomers can optionally also be copolymerized. Examples of auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid; ethylenically unsaturated carboxylic acid amides and nitriles, preferably acrylamide and acrylonitrile; mono- and diesters of fumaric acid and maleic acid, such as the diethyl and diisopropyl esters, and maleic anhydrides; and ethylenically unsaturated sulphonic acids and salts thereof, preferably vinylsulphonic acid and 2-acrylamido-2-methylpropanesulphonic acid. Further examples are pre-crosslinking comonomers, such as poly-ethylenically unsaturated comonomers, for example divinyl adipate, diallyl maleate, allyl methacrylate or triallyl cyanurate. Epoxide-functional comonomers, such as glycidyl methacrylate and glycidyl acrylate, are also suitable. Further examples are silicon-functional comonomers, such as acryloxypropyltri(alkoxy)- and methacryloxypropyltri-(alkoxy)-silanes, vinyltrialkoxy silanes and vinylmethyldialkoxy silanes, which can contain, for example, ethoxy and ethoxypropylene glycol ether radicals as alkoxy groups. Monomers with hydroxyl or CO groups, for example methacrylic acid and acrylic acid hydroxyalkyl esters, such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate, and compounds such as diacetoneacrylamide and acetylacetoxyethyl acrylate or methacrylate, may also be mentioned.
The choice of monomer and the choice of the weight contents of the comonomers is taken here such in general a glass transition temperature Tg of xe2x88x9270xc2x0 C. to +100xc2x0 C., preferably xe2x88x9250xc2x0 C. to +50xc2x0, particularly preferably xe2x88x9220xc2x0 C. to +20xc2x0, results. The glass transition temperature Tg of the polymers can be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg can also be calculated beforehand in approximation by means of the Fox equation. According to Fox T. G., Bull. Am. Physics Soc. 1, 3, page 123 (1956): 1/Tg=x1/Tg1+x2/Tg2+ . . . +xn/Tgn, wherein xn represents the weight fraction (% by weight/100) of monomer n and Tgn is the glass transition temperature in degrees Kelvin of the homopolymer of monomer n. Tg values for homopolymers are listed in Polymer Handbook 2nd Edition, J. Wiley and Sons, New York (1975).
Suitable protective colloids are, for example, polyvinyl alcohols, polysaccharides in water-soluble form, such as starches (amylose and amylopectin), modified starches, such as starch ethers, for example hydroxyalkyl ether starches, dextrins and cyclo-dextrins, celluloses and carboxymethyl, methyl, hydroxyethyl and hydroxypropyl derivatives thereof, poly(meth)acrylic acid, poly(meth)acrylamide, melamine-formaldehyde sulphonates and naphthalene-formaldehyde sulphonates.
Polyvinyl alcohols with a degree of hydrolysis of 80 to 95 mol % and a Hxc3x6ppler viscosity in 4% strength aqueous solution of 1 to 30 mPas (Hxc3x6ppler method at 20xc2x0 C., DIN 53015) are preferred. Hydrophobically modified polyvinyl alcohols with a degree of hydrolysis of 80 to 95 mol % and a Hxc3x6ppler viscosity in 4% strength aqueous solution of 1 to 30 mPas are also suitable. Examples of these are partly hydrolysed copolymers of vinyl acetate with hydrophobic comonomers, such as isopropenyl acetate, vinyl pivalate, vinyl ethylhexanoate, vinyl esters of saturated alpha-branched monocarboxylic acids having 5 or 9 to 11 C atoms, dialkyl maleates and dialkyl fumarates, such as diisopropyl maleate and diisopropyl fumarate, vinyl chloride, vinyl alkyl ethers, such as vinyl butyl ethers, and olefins, such as ethene and decene. The content of the hydrophobic units is preferably 0.1 to 10% by weight, based on the total weight of the partly hydrolysed polyvinyl acetate. Mixtures of the polyvinyl alcohols mentioned can also be employed.
The partly hydrolysed polyvinyl acetates with vinyl alcohol units and units of vinyl esters of alpha-branched carboxylic acids having 5 or 9 to 11 C atoms in the amounts mentioned are particularly preferred. Examples of such vinyl esters are those which are available as versatic acid vinyl esters from Shell under the names VeoVaR5, VeoVaR9, VeoVaR10 and VeoVaR11. Further suitable polyvinyl alcohols are partly hydrolysed, hydrophobized polyvinyl acetates, which are obtained by polymer-analogous reaction, for example acetalization, of the vinyl alcohol units with C1- to C4-aldehydes, such as butyraldehyde. The content of the hydrophobic units is preferably 0.1 to 10% by weight, based on the total weight of the partly hydrolysed polyvinyl acetate. The degree of hydrolysis is from 80 to 95 mol %, preferably 85 to 94 mol %, and the Hxc3x6ppler viscosity (DIN 53015, Hxc3x6ppler method, 4% strength aqueous solution) is from 1 to 30 mPas, preferably 2 to 25 mPas.
Polyvinyl alcohols with a degree of hydrolysis of 85 to 94 mol % and a Hxc3x6ppler viscosity in 4% strength aqueous solution of 2 to 25 mPas (Hxc3x6ppler method at 20xc2x0 C., DIN 53015) and combinations thereof with the hydrophobically modified polyvinyl esters mentioned in a weight ratio of 10/1 to 1/10 are most preferred. The protective colloids mentioned are accessible by means of processes known to the expert.
The polymer powders stabilized with protective colloids are prepared by the emulsion polymerization process, the polymerization temperature in general being 40xc2x0 C. to 100xc2x0 C., preferably 60xc2x0 C. to 90xc2x0 C. The copolymerization is usually carried out under pressure, in general between 3 bar and 10 bar. The polymerization is initiated with the initiators or redox initiator combinations customary for emulsion polymerization, for example hydroperoxides, such as tert-butyl hydroperoxide, azo compounds, such as azobisisobutyronitrile, and inorganic initiators, such as the sodium, potassium and ammonium salts of peroxodisulphuric acid. The initiators mentioned are in general employed in an amount of 0.05 to 3% by weight, based on the total weight of the monomers. Redox initiators which are used are combinations of the initiators mentioned with reducing agents, such as sodium sulphite, sodium hydroxymethanesulphinate and ascorbic acid. The amount of reducing agent is preferably 0.01 to 5.0% by weight, based on the total weight of the monomers.
The polymerization batch is stabilized by means of the protective colloids mentioned, without additional emulsifiers. Preferably, some of the protective colloid content is initially introduced here into the reaction vessel, and some is metered in after initiation of the polymerization. The polymerization is in general carried out in the presence of 1 to 25% by weight of protective colloid, based on the total weight of the monomers. The monomers can be initially introduced in total, metered into the reaction vessel in total or initially introduced in portions, with the remainder being metered in after the initiation of the polymerization. The content of post-crosslinking comonomers c) is preferably metered into the reaction vessel in total.
After conclusion of the polymerization, after-polymerization can be carried out using known methods, for example by after-polymerization initiated with a redox catalyst, for removal of the residual monomers. Volatile residual monomers can also be removed by means of distillation, preferably under reduced pressure, and optionally by passing inert entraining gases, such as air, nitrogen or steam, through or over the mixture. The aqueous dispersions obtainable by this process in general comprise 1 to 25% by weight of protective colloid, based on the polymer content, and have a solids content of 30 to 75% by weight, preferably 40 to 65% by weight.
To prepare the water-redispersible polymer powders, the aqueous dispersions are dried, for example by means of fluidized bed drying, freeze drying or spray drying. The dispersions are preferably spray-dried. Spray drying is carried out here in customary spray-drying units, it being possible for the atomization to take place by means of one-, two- or multi-component nozzles or with a rotating disc. The exit temperature is in general chosen in the range from 55xc2x0 C. to 100xc2x0 C., preferably 70xc2x0 C. to 90xc2x0 C., depending on the unit, the Tg of the resin and the desired degree of drying.
The total amount of protective colloid before the drying operation should preferably be at least 10% by weight, based on the polymer content. To ensure the redispersibility, as a rule it is necessary to add further protective colloids to the dispersion before drying, as spraying aids. The content of protective colloid before spraying of the dispersion is as a rule 5 to 25% by weight, based on the polymeric constituents of the dispersion.
Suitable spraying aids are partly hydrolysed polyvinyl acetates; polyvinyl pyrrolidones; polysaccharides in water-soluble form, such as starches (amylose and amylopectin) and modified starches, such as starch ethers, for example hydroxyalkyl ether starches; celluloses and carboxymethyl, methyl, hydroxyethyl and hydroxypropyl derivatives thereof; proteins, such as casein or caseinate, soya protein and gelatin; lignin-sulphonates; synthetic polymers, such as poly(meth)acrylic acid, copolymers of (meth)acrylates with carboxyl-functional comonomer units, poly(meth)acrylamide, polyvinylsulphonic acids and water-soluble copolymers thereof; and melamine-formaldehyde sulphonates, naphthalene-formaldehyde sulphonates and styrene-maleic acid and vinyl ether/maleic acid copolymers. Partly hydrolysed polyvinyl acetates with a degree of hydrolysis of 80 to 95 mol % and a Hxc3x6ppler viscosity of 1 to 30 mpas, which can optionally be hydrophobically modified as described above, are preferred as spraying aids.
During the spraying, a content of up to 1.5% by weight of antifoam, based on the base polymer, has proved favourable in many cases. To increase the storage stability by improving the stability to blocking, in particular in the case of powders with a low glass transition temperature, an antiblocking agent (anticaking agent), preferably in an amount of up to 30% by weight, based on the total weight of the polymeric constituents, can be added to the resulting powder. Examples of antiblocking agents are Ca carbonate and Mg carbonate, talc, gypsum, silicic acid and silicates with particle sizes preferably in the range from 10 nm to 10 xcexcm.
To increase the resistance of the glue to water, 0.5 to 10% by weight, based on the polymer, of water-soluble metal salts of monobasic acids with a pKa of less than 0 can also be added to the polymer dispersions or polymer powders stabilized with protective colloids. Salts of Al(III), Fe(III) and Zr(IV), for example aluminium nitrate, iron chloride and zirconium oxychloride, are particularly suitable. Free acids with a pKa of less than 0 can also be employed in the same amounts as hardeners. Examples of these are hydrochloric acid, nitric acid, perchloric acid, phosphoric acid or mixtures thereof. Crosslinking agents can also be added in an amount of 0.5 to 10% by weight, based on the polymer, to further improve the resistance to water. Examples of these are polyisocyanates, formaldehyde-phenol resins or masked polyaldehydes, for example bisulphite adducts of dialdehydes, such as glutaradialdehyde-bis-sodium hydrogen sulphite.
The adhesives are suitable for gluing porous substrates. Examples of this are the gluing of wood, such as wood-wood gluing, and the gluing of wood to absorbent substrates, such as plaster, in particular gluing of parquet. Further uses are waterproof gluing of paper and cardboard, for example as a packaging adhesive and book-binding adhesive. The adhesives are also suitable for gluing fibre materials of natural or synthetic fibre, for example for the production of wood fibreboard, for consolidation of nonwovens of natural or synthetic fibres, for the production of mouldings from fibre materials and for the production of precursors of such mouldings, the so-called semi-finished products (waddings). Further examples of use are the binding of wadding, for example of cushion, insulating and filter wadding, and the production of laminates, such as insulating materials.